Solvent borne air drying coatings have been used on substrates such as wood and metal for many years. Generally, they are used both to give protection to the substrate and to provide an aesthetically pleasing appearance. Depending upon the nature of the polymeric binder used in the final coating, these decorative air drying coatings can be applied to both interior and exterior surfaces.
For both consumer and professional applications, the predominant organic solvent is conventionally an aliphatic hydrocarbon such as white spirit, which contains around 18% aromatic materials. More recently, white spirit has been replaced with low aromatic analogues which have the benefit of reducing the toxicity of the solvent and at the same time, reducing odour. Other organic solvents such as esters and ketones are often present in small quantities, usually from such additives as driers, fungicides and tinters.
Despite the move towards less toxic and lower odour solvents, the fact that organic solvents are used, still results in their evaporation into the atmosphere after application, thus contributing to atmospheric pollution.
A reduction in the level of volatile organic solvents used in decorative coatings is therefore considered desirable, indeed necessary. The development of high solids paints, with much lower levels of organic solvent, would have a significant impact on atmospheric pollution. However, the introduction of such products has been delayed due to the lack of enforcing legislation and their relatively higher price.
An alternative to the organic solvent as a mobile carrier for the resin, pigment etc., is water. Water based decorative coatings have been available for many years and the most common types are generally formulated on thermoplastic (co)polymers derived from monomers such as vinyl acetate, vinyl acetate/Veova, vinyl acetate/ethylene copolymers, styrene, styrene/methacrylates. In these cases, the polymer is formed by emulsion polymerisation in the aqueous phase, to produce an emulsion with dispersed discrete particles. Polymers formed by this process usually have a very high molecular weight. In order for coatings to be formed from these polymers, the particles must coalesce to form a coherent resistant film. Due to the nature of the polymerisation process, water sensitive materials remain in the thermoplastic coating.
Air drying water borne resins have also been available for many years. These may be alkyds with high acid value. Traditionally, this acidity has been achieved by ring opening of trimellitic anhydride by reaction with a hydroxyl terminated alkyd. The alkyd may then be thinned to about 70% to 80% in a water miscible organic solvent such as butyl glycol and neutralized with ammonia or amine, to render the product water dilutable. These materials have never achieved commercial success in the decorative paint market, due to high solvent content and poor application and performance properties.
Alkyd emulsions are another route to producing water based decorative coatings. It is reasonable to assume that these will retain their oxidative nature and so crosslink after application. However, they have not gained a large commercial success in decorative paints and varnishes, due to performance related problems, such as yellowing, drying, drier stability, water resistance and poor rheology.
Therefore, there is still a need for an autoxidisable binder for water-based coatings which can produce finished coatings with higher abrasion resistance and/or hardness, are preferably also faster-drying and in the case of the known high-acid resins, contain less organic solvent.
One preferred sub-class of binders according to the invention comprises those which result in thixotropic coating compositions. Therefore, it is convenient here to review the prior art relating to thixotropic system.
The solvent based coatings widely used in the domestic consumer market are generally thixotropic in appearance. That is to say that the paint exhibits time dependent recovery. In the undisturbed state at low shear rates, these coatings, which can be paints, varnishes or stains, have a high apparent viscosity. The actual low shear viscosity will depend upon the degree of structure in the coating but in a typical non-drip coating the low shear viscosity would be in the order of 1 000 000 mPa.s. This has the effect of making the coating appear like a jelly in the indisturbed state in the can before application. At high shear rates, typically those experienced when the paint is brushed out, the paint will exhibit the viscosity characteristics of a liquid paint thus allowing easy application. Once brushing is stopped, the paints will show a recovery in viscosity and structure with time. This will allow the paints to flow and level on the substrate without sagging, thus giving a coating which is uniform in thickness and virtually free from brush marks.
This thixotropic character can conveniently be measured using a constant stress rheometer and undertaking an oscillation recovery sweep. The parameters measured are the elastic modulus G' and the viscous modulus G". Immediately after applying a shearing force, the viscous modulus will dominate and the paint will flow. As time passes, both G' and G" show an increase. With a thixotropic material, the rate of increase in the elastic modulus will be faster than G" and will eventually overtake the viscous modulus, at which time the product can be considered to be more solid in nature and will now no longer sag.
Any product which after a period of time after applying a shearing force will show a curve in which G' increases at a faster rate than G" can be considered thixotropic.
The truly thixotropic solvent based coatings, widely available in the domestic consumer market, are usually based on autoxidisable binders which have been chemically modified by polyamide technology or urethane/urea technology as described in GB-A-1,454,388 and GB-A-1,454,414.
Other known means of imparting structure into solvent based coatings, which is not truly thixotropic, is by means of clays, silicas, amide additives, hydrogenated castor oil additives.
The water based binders described above, whether they are the thermoplastic emulsion (co)polymers or alkyd emulsions, which are widely used in the domestic decorative market, are not inherently thixotropic. Any structure, which these materials have, is imparted by the addition of additives at the paint manufacturing stage and not by chemical modification of the binder itself.
There have been other proposals for thixotropic systems which are aqueous, for example utilising the binder disclosed in GB-A-2,237,576. It is based on an acrylic polymer having hydroxyl and carboxylic acid pendant groups. This polymer material is made thixotropic by reaction with an isocyanate having at least two isocyanate groups, an amine having at least two amino groups and a primary or secondary monoamine. However, although these polymers are water-dispersed, their ability to post-cure, and therefore their final film performance is limited.
Thus, to date, there remains a need for a commercially viable water based coating system and in particular, a binder therefore, which system can be produced in thixotropic or non-thixotropic form as required.